These pranks I once did for fun, but maybe it can be used as an exercises.
Internal calculator: plus and minus are swapped with each other. Despite the fact that AutoCAD is a formidable and old software, it was easy to do.
I think, a practicing reverse engineer should spend several hours on this and do this without effort. Beginners may try as well. I spent one day.
Wolfram Mathematica is a formidable software too, but it was easy, I spent only ~1-2 days on this problem.
Not that MATLAB is very different from Wolfram Mathematica, but I had to spend about a week on this. If you think you're a good reverser, you should do this as well.
This is hardest. I spent only couple of days on this, but I had a prior experience, I knew a lot of (internal) details about it before.
This probably should not be used as an exercise -- Oracle RDBMS is really a highly complicated thing (no joke). However, a practicing reverse engineer should have at least one quick good idea where to start. (I had two working ideas.)
If you think you're good, you should have 1-2 ideas as well.
If you want to check your ideas, you can just ask me: dennis at yurichev dot com. You can use temporary/anonymous/throwaway/sock puppet email(s): I don't care.
After several months, I'll post more details about these exercises, stay tuned, as they say. Also, I'll do full write-ups about how I did it all.
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